1. Field of the Invention
The present invention relates to a method for producing an electrophotographic photoreceptor, and to an electrophotographic photoreceptor and an image forming apparatus.
2. Description of the Related Art
An electrophotographic photoreceptor (hereinafter this may be simply referred to as “photoreceptor”) has a constitution of a photoconductive substance-containing photosensitive layer laminated on a conductive substrate. One example of the electrophotographic photoreceptor is an inorganic photoreceptor in which the photosensitive layer comprises an inorganic photoconductive substance such as selenium, zinc oxide or cadmium sulfide as the essential ingredient thereof.
The basic characteristics necessary for the photoreceptor are that it has good electric properties, for example, it has good charge-retentive capability and discharges little in a dark place, it has good photosensitivity and readily discharges through exposure to light. The others also necessary for the photoreceptor are that its electric properties as above are stable even in repeated use so that it can form homogeneous images for a long period of time, and it has good electric characteristic stability (this may be hereinafter simply referred to as “characteristic stability”).
The inorganic photoreceptor may have such basic properties of photoreceptor, but has some drawbacks in that its photosensitive layer is difficult to form, its surface is poorly plastic and is readily scratched, and its production costs are high. In addition, the inorganic photoconductive substance mentioned above is highly toxic and is seriously limited in point of its production and use.
In place of the inorganic photoreceptor using such an inorganic photoconductive substance, an organic photoreceptor using an organic photoconductive substance is being studied and developed. An organic photoconductive substance is being widely studied and developed, and it is utilized not only in electrostatic recording devices such as electrophotographic photoreceptors but also applied to sensors, organic electroluminescent devices, etc.
The organic photoreceptor using an organic photoconductive substance has the advantages that the photosensitive layer is easy to form and is highly plastic, and that the layer is lightweight and its transparency is good. In addition, another advantage of the organic photoreceptor is that, according to a method of adding a chemical sensitizer or an optical sensitizer thereto, the photoreceptor may be made to generate free charges to light having a broad wavelength range and its photosensitivity may be thereby bettered. Though the organic photoreceptor has one problem in that its durability is somewhat poorer than that of the inorganic photoreceptor, it is the mainstream of electrophotographic photoreceptors as having the above-mentioned various advantages.
As one type of the organic photoreceptor, a function-separated electrophotographic photoreceptor has been developed, of which the charge-generating function and the charge-transporting function are separately attained by different substances. In the function-separated electrophotographic photoreceptor of the type, different substances are used for the charge-generating substance that bears the charge-generating function thereof and for the charge-transporting substance that bears the charge-transporting function thereof. Accordingly, the materials to be used for the charge-generating substance and for the charge-transporting substance may be readily selected, and the photosensitivity of the organic photoreceptor may be thereby further bettered, and therefore electrophotographic photoreceptors having any desired properties may be produced relatively easily.
The function-separated photoreceptor is grouped into two; one is a single-layered photoreceptor having a photosensitive layer in which a charge-generating substance and a charge-transporting substance are co-dispersed in a resin having a binding capability that is referred to as a binder resin, and the other is a laminate-structured photoreceptor having a laminate photosensitive layer that comprises a charge-generating layer with a charge-generating substance dispersed therein and a charge-transporting layer with a charge-transporting substance dispersed therein. Both the single-layered photoreceptor and the laminate-structured photoreceptor are produced according to a process that comprises a coating step of applying a coating liquid, which is prepared by dissolving or dispersing a photosensitive layer-forming material that comprises an organic photoconductive substance including a charge-generating substance or a charge-transporting substance, and a resin having a binding capability that is referred to as a binder resin, in a solvent capable of dissolving the binder resin, onto a conductive substrate to form thereon a layer having a uniform thickness (hereinafter the coating liquid applied to the conductive substrate is referred to as “coating film”), and a drying step of drying the coating film to thereby remove the solvent having remained in the coating film.
For the laminate-structure photoreceptor, much used is a normal two-layered photoreceptor that comprises a charge-generating layer formed on the side of the conductive substrate and a charge-transporting layer formed on the outer surface side of the photoreceptor, or that is, on the opposite side to the conductive substrate relative to the charge-generating layer. In the normal two-layered photoreceptor, the charge-transporting layer is laminated on the surface of the charge-generating layer, or that is, on the opposite side to the conductive substrate relative to the charge-generating layer, and the charge-transporting layer has a hole-transporting function. Therefore, the photoreceptor of the type is sensitive to light while it is charged negatively, and it is essentially used under negative charge. On the other hand, a reverse two-layered photoreceptor has been developed as a laminate-structure photoreceptor usable under positive charge, in which a charge-transporting layer is formed on the conductive substrate side of the photoreceptor and a charge-generating layer is formed on the surface side thereof.
As in the above, various types of organic photoreceptors have been proposed. These organic photoreceptors may satisfy various requirements in a broad range, but have some problems common to them in that the characteristic stability of organic photoreceptors is insufficient. Organic photoreceptors undergo fatigue degradation in repeated use, such as charge potential reduction or residual potential increase. As a result, an image forming apparatus that comprises such an organic photoreceptor may have some problems of resolution reduction and image failure of white skipping or dark stripes. White skipping means a phenomenon of such that toner does not adhere to a part to which it should adhere. Dark stripes mean a phenomenon of such that toner adheres to a part to which it should adhere and erroneously even to another part except it, like stripes.
The fatigue degradation of a photoreceptor to worsen its characteristic stability may be caused by a charger used as a charging unit for charging the photoreceptor. Concretely, a photoreceptor may be fatigued and degraded by an oxidizing gas such as ozone generated through corona discharge by a charger driven according to a corona discharge system (hereinafter, this may be referred to as “corona-discharge charger”) and nitrogen oxide to be formed through reaction of the generated ozone with nitrogen in air. The oxidizing gas oxidizes the photoconductive substance that exists on and around the outer surface of a photoreceptor having a photosensitive layer formed on the outer surface side thereof. When the photoreceptor is repeatedly used and when the photoconductive substance is oxidized, then the photosensitive layer may be fatigued and degraded through the oxidation of the photoconductive substance, and the life of the photoreceptor is thereby shortened.
For solving the problem of fatigue degradation of the photosensitive layer, one method is proposed, which comprises providing an exhaust unit inside an image forming apparatus on which a photoreceptor is mounted, and exhausting the oxidizing gas around the corona-discharge charger. Providing an exhaust unit inside an image forming apparatus in that manner may make it possible to exhaust the oxidizing gas around the corona-discharge charger and to prevent the oxidation of the photoconductive substance, but this is problematic in that the constitution of the image forming apparatus may be complicated.
Another method has been tried, which comprises using a bisphenol Z-type polycarbonate resin having an excellent gas-barrier capability as the binder resin for a photosensitive layer, thereby improving the gas-barrier property of the surface of the photosensitive layer and improving the oxidizing gas transmission resistance of the photosensitive layer so as to prevent the fatigue degradation of the photosensitive layer. However, a photosensitive layer capable of exhibiting a sufficient gas-barrier property not detracting from the electric properties of the photoconductive substance in the photosensitive layer has not as yet been realized.
For preventing the oxidation by an oxidizing gas of the photoconductive substance existing around the surface of a photosensitive layer, a method has been proposed, which comprises adding an antioxidant, a stabilizer or the like to the photosensitive layer. For example, proposed is adding a hindered phenol-type antioxidant such as a compound having a triazine ring and a hindered phenol skeleton, to a photosensitive layer (e.g., see Japanese Unexamined Patent Publication JP-A 62-105151 (1987)).
As another prior-art technique, proposed is adding, as an additive, a hindered phenol-type antioxidant of a hindered phenol skeleton-having compound, or a phosphite-type antioxidant or an amine-type antioxidant, to a photosensitive layer that contains a specific arylamine compound (e.g., see Japanese Unexamined Patent Publication JP-A 8-292587 (1996)). As still another prior-art technique, proposed is adding a hindered amine skeleton-having compound and a specific structure-having amine compound such as tribenzylamine to a photosensitive layer (e.g., see JP-A 8-292587).
The hindered phenol skeleton-having compound disclosed in these prior-art references is a phenol compound having a bulky substituent such as a branched alkyl group, a cycloalkyl group, an aryl group or a heterocyclic group, in the position adjacent to the phenolic hydroxyl group therein. The hindered amine structure-having compound is an amine compound in which the hydrogen atom of the amino group is substituted with a bulky substituent such as a branched alkyl group, a cycloalkyl group, an aryl group or a heterocyclic group.
When the antioxidant disclosed in JP-A 62-105151, JP-A 8-292587, Japanese Unexamined Patent Publication JP-A 10-282696 (1998) is added to a photosensitive layer, then the oxidation of the photoconductive substance in the layer by an oxidizing gas may be prevented and the fatigue degradation of the photoreceptor may be thereby prevented. However, when the antioxidant is added to the photosensitive layer in such a degree that the oxidation of the photoconductive substance can be thereby prevented, then there may occur some problems in that the photosensitivity of the photoreceptor is lowered or the photo-responsiveness thereof is lowered owing to the change in the sensitive wavelength range, and while the photoreceptor is used repeatedly in an electrophotographic process comprising a series of charging, exposure and discharging, its charge potential may lower and its residual potential may increase. In order to evade these problems, when the amount of the antioxidant to be added is reduced, then the oxidation of the photoconductive substance could not be sufficiently prevented. To that effect, the prior-art techniques could not realize a photoreceptor capable of satisfying both good electric properties and good resistance to oxidizing gas.
To solve these problems, proposed is another technique of making a hindered amine skeleton-having compound (hereinafter, this may be referred to as hindered phenol compound) serving as an antioxidant exist in the surface of a photosensitive layer, not adding such an antioxidant to the constitutive material itself of the photosensitive layer (e.g., see JP-A 2-146556 (1990)). The technique disclosed in JP-A 2-146556 (1990) is to propose forming a hindered amine compound-containing protective layer on the surface of a photosensitive layer, by supplying a hindered amine compound to the surface of a photosensitive layer through contact of a contact member such as an elastic roller or an elastic blade with a photoreceptor. According to the technique of making an antioxidant exist in the surface of a photosensitive layer in the manner as above, the photoconductive substance existing in the photosensitive layer around the outer surface of the photoreceptor may be prevented from being oxidized by an oxidizing gas, and the characteristic stability of the photoreceptor may be thereby prevented from being lowered. In addition, since the photosensitive layer itself does not contain an antioxidant, the electric properties of the photoreceptor do not lower.
The technique disclosed in JP-A 2-146556 (1990) may be effective for preventing the oxidation of a photoconductive substance by an oxidizing gas, but has the following problems. For example, in case where a hindered amine compound is supplied to the surface of a photoreceptor through contact of a contact member such as an elastic roller or an elastic blade with a photoreceptor, the elastic material to constitute the contact member must contain a hindered amine compound. The process of producing such a contact member is complicated, and, depending on the contact pressure between the contact member and a photoreceptor, a sufficient amount of a hindered amine compound enough for the intended oxidizing gas resistance could not be supplied to the surface of the photoreceptor. When the contact pressure between the contact member and the photoreceptor is increased in order that a sufficient amount of a hindered amine compound could be supplied to the surface of the photoreceptor, then the photosensitive layer, or that is, the surface layer of the photoreceptor may be worn by the contact pressure and the thickness of the photosensitive layer may be thereby reduced, and there may occur other problems in that the electric properties of the layer may be worsened and the life of the photoreceptor may be shortened.
When a hindered amine compound is added to the protective layer provided on the surface of a photosensitive layer, then the protective layer must be indispensably formed in a photoreceptor with the result that the types of the coating liquids to be used in producing a photoreceptor increase. In forming a photoreceptor, the viscosity of the coating liquid is an important physical factor for defining the thickness dimension of the photosensitive layer, and it is an indispensable condition to highly accurately control the viscosity of the coating liquid for efficiently producing a photoreceptor. Accordingly, increasing the types of the coating liquids results in other various problems in that the control of the coating liquids is complicated, the plant investment for viscosity control increases, and the production costs increase in point of the material such as the solvent to be used for the viscosity control since a fresh solvent must be replenished for compensating the solvent that may reduce through vaporization.
To solve these problems, desired is a simple method for increasing the resistance of the photosensitive layer to an oxidizing gas and for preventing the reduction in the characteristic stability of a photoreceptor, not increasing the types of the necessary coating liquids relative to the number of the photosensitive layers to be formed and not detracting from the electric properties of the photoreceptor.